Circuit arrangement and actuation method for semi-conductor light sources

ABSTRACT

A circuit arrangement for driving semiconductor light sources is provided. The circuit arrangement may include video electronics, which are configured to generate a timing signal and an image value table; a driver circuit, which comprises a digital and an analogue part, and a semiconductor light source module, which can contain one or a plurality of semiconductor light sources; and a switch being arranged between the driver circuit and the semiconductor light source module, which switch is driven by the driver circuit and can switch the desired value for the power supply for the semiconductor light source module away from an analogue/digital converter to logic 0.

TECHNICAL FIELD

The invention relates to projection devices equipped with semiconductorlight sources. Microdisplay applications for front and rear projectioncan primarily be mentioned here.

PRIOR ART

Recently, powerful semiconductor light sources such as high-power lightemitting diodes have increasingly been used in applications which hadpreviously been reserved for high-pressure discharge lamps. Precisely inthe field of projection, the semiconductor light sources are not drivencontinuously, but rather operated in pulsed fashion in order to meet therequirements there. In that case, very short pulses are employed, whichin return have a very high power density.

A plurality of colors, usually at least 3 colors, are used in displayapplications. The drive circuit is divided into video electronics, whichcondition the input signal, and driver electronics, which are oftensplit into a digital and an analogue part connected via adigital/analogue converter. In accordance with the input signal, thevideo electronics pass instructions to the driver electronics to switchon a specific color. In the digital part of the driver electronics, aprocess is thereupon initiated which, after a specific time, results inthe outputting of a brightness value for the desired color. Saidbrightness value and the timing are held in a table generated by thevideo electronics. In response to the request of the video electronics,the so-called strobe, the digital driver electronics read out said tableand generate a desired value signal on the basis of the values read out.The analogue driver part thereupon switches on the corresponding coloredsemiconductor light source with the desired brightness. Since the colorsare projected successively in display applications, the semiconductorlight sources have to be switched off again after the pulse that hasbeen output in order that the next color can be projected. This requiresa further cycle in the digital driver electronics. A zero value has tobe input into the buffer memory of the analogue/digital converter andsaid value then has to be output as analogue value, whereupon theanalogue part switches off the semiconductor light source again.

Therefore, a certain time elapses between the strobe signal and theactual switching on or off of the semiconductor light source. In thecase of switching on the semiconductor light sources this does not posea problem since this delay time is known to the video electronics andconcommitently taken into account in the timing. In the case ofswitch-off, however, this can pose a problem since the minimumswitched-on duration is dependent on the processing speed for the driverelectronics. For this reason, the driver electronics must have a fastlogic and fast analogue/digital converter since otherwise they cannotachieve the required processing speed. However, fast analogue/digitalconverters and a fast logic (e.g. a highly clocked microcontroller) arevery cost-intensive, which has a disadvantageous effect on the pricingof the end product.

Object

Therefore, it is an object of the invention to propose a drive methodand a circuit arrangement which no longer have the disadvantagesmentioned above and manage with a slower and therefore morecost-effective logic.

SUMMARY OF THE INVENTION

This object is achieved by means of the features of patent claim 1 andof method claim 7. Particularly advantageous embodiments of theinvention are described in the dependent claims.

The invention is based on the fact that the semiconductor light sourcesis always switched off after the pulsed light emission. This makes itpossible to input a new value into the analogue/digital converter onlyonce and to switch the semiconductor light source on and off by means ofa switch. This has the advantage that slow components can be usedwithout restricting the performance of the overall arrangement.

BRIEF DESCRIPTION OF THE DRAWING(S)

FIG. 1 Block diagram of a circuit arrangement according to the priorart.

FIG. 2 Block diagram of a circuit arrangement according to theinvention.

FIG. 3 Comparison of some relevant signal profiles according to theprior art and the present invention.

PREFERRED EMBODIMENT OF THE INVENTION

The block diagram of a circuit arrangement according to the prior art isshown in FIG. 1. The digital part of driver electronics 1 contains atiming signal decoding logic 13, and a control logic 11 for thedigital/analogue converter 7. The analogue part comprises adigital/analogue converter 7, a control logic 3 and the light emittingdiode modules 5. The light emitting diode modules 5 include the powerdrivers for the LEDs and the LEDs themselves. The analogue brightnesssignal is input into the power drivers, which signal then switches onthe LEDs to the desired brightness level or, in the case of an analoguesignal corresponding to logic zero, switches them off. The timing signaldecoding logic receives the timing signal (strobe) from the videoelectronics (not shown). The timing signal is processed, and the timingsignal decoding logic outputs a start signal to the control logic 11 atthe correct point in time. Said logic reads out a table which isgenerated by the video electronics and in which timing and brightnessvalues for the various phases of an image are stored. The sequence whichthen follows can thus be seen from FIG. 3. The upper third depicts thetiming signals generated by the video electronics, and the middle thirddepicts the actions and signals of a circuit arrangement according tothe prior art.

The block 21 represents the read-out of the table. This begins at theinstant T₁, as soon as the timing signal is logic 1. As soon the valuesfrom the table have been determined, they are written to the buffermemory of the digital/analogue converter (block 23). In block 25, assoon as the buffer memory has been completely updated, thedigital/analogue converter is put into action, with the result that ananalogue signal corresponding to the digital value is present at theanalogue output. The driver circuit requires the time t_(LOG1) for allthis. As soon as the analogue signal is output, the analogue part of thedriver circuit switches on the LED modules 5. This process requires thetime t_(DACset1).

At the instant T₃, the timing signal becomes logic zero again. At thisinstant, the entire procedure is run through a further time in order toset the analogue value to zero again and thus to switch off the LEDmodule. The time between the instants T₁ and T₂ is equal to the timingsignal duration t_(STB). The processing time t_(LOG1) is cruciallyresponsible for the minimum pulse duration of the entire arrangement,because if the duration of the processing time t_(LOG1) comes close tothe minimum timing signal duration t_(STB.min), the timing can no longerbe complied with. The minimum pulse length and thus the minimum timingsignal duration t_(STB.min) are therefore directly dependent on theprocessing speed of the driver circuit.

This is the starting point for the invention. According to theinvention, the analogue/digital converter is used only for setting thebrightness, with the result that, for each color, said converter nowonly has to run through the conversion procedure once instead of twice.A switch is arranged between the analogue output downstream of thecontrol logic 3 and the LED module, which switch either switches theanalogue value of the analogue/digital converter through the LED moduleor switches the analogue input of the LED module to ground, whichcorresponds to a logic zero. The signals and actions of a drive methodaccording to the present invention are shown in the lower third of FIG.3. As can be seen from the length of the logic blocks 21-25, theprocessing time is significantly longer, that is to say that theprocessing speed is lower. This results from the use of morecost-effective components that operate more slowly.

The signal 51 describes the output of the digital/analogue converter.The latter is switched over as soon as the digital processing isconcluded. This takes the time t_(LOG2), which is longer than the timet_(LOG1). As soon as the analogue/digital converter has reliablyswitched over, the switch S_(X) of the corresponding color is switchedon (signal 53). The power driver thus receives the analogue signal 51from the digital/analogue converter and switches on the LEDs (signals55, 57).

After the time t_(STB), that is to say the pulse duration, the switch isswitched to ground again, whereupon the LEDs are switched off. In thiscase, the analogue/digital converter remains at its original value, thatis to say that the analogue/digital converter signal is not brought tozero. For the next cycle, the analogue/digital converter is merely setto the new brightness.

By virtue of the fact that the analogue/digital converter only operatesfor setting the brightness, the processing time is significantly lesscritical. The analogue/digital converter, after all only operates beforethe pulse, that is to say that the delay time of the converter isunimportant for the minimum pulse length. The invention, on thecontrary, has the major advantage of being able to realize very shortminimum pulse lengths, since practically no processing has to take placeduring the pulse, rather the majority of the processing can take placeduring the pulse intermissions. Since the longer processing time beforethe pulse can easily be compensated for by the video electronics, thedrive method according to the invention has no disadvantages whatsoevereven in the case of components operating more slowly.

The time available to the driver electronics to accomplish other thingsas can be seen from block 27, is likewise lengthened by comparison withthe prior art. A further advantage is that this time can occur duringthe pulse and during the pulse intermissions.

1. A circuit arrangement for driving semiconductor light sources,comprising: video electronics, which are configured to generate a timingsignal and an image value table; a driver circuit, which comprises adigital and an analogue part, and a semiconductor light source module,which can contain one or a plurality of semiconductor light sources; anda switch being arranged between the driver circuit and the semiconductorlight source module, which switch is driven by the driver circuit andcan switch the desired value for the power supply for the semiconductorlight source module away from an analogue/digital converter to logic 0.2. The circuit arrangement as claimed in claim 1, wherein the digitalpart of the driver circuit comprises a microcontroller.
 3. The circuitarrangement as claimed in claim 2, wherein the switch is driven by atiming signal decoding logic.
 4. The circuit arrangement as claimed inclaim 2, wherein all the switches are controlled jointly by an output.5. The circuit arrangement as claimed in claim 2, wherein each switch iscontrolled individually by an output.
 6. The circuit arrangement asclaimed in claim 2, wherein the switches are controlled individually byan output, wherein the output is configured to output a coded signal,and the switch drive circuit has a decoding logic that is configured toswitch the respective switch on and off on the basis of the codedsignal.
 7. A drive method for driving semiconductor light sources, themethod comprising: video electronics generating a timing signal; adriver circuit, which comprises a digital and an analogue part,outputting an analogue brightness signal that is input into a powerdriver; wherein the input of the power driver can be switched from ananalogue/digital converter to ground.
 8. The drive method as claimed inclaim 7, wherein the switch-over is realized by a timing signal decodingunit.